19 — hydrocarbons

Question 1

Homologous series

Answer 1

Consists of a family of compounds with the same general formula, and similar chemical properties because they have the same functional group

Question 2

Alkanes

Answer 2

Alkanes are hydrocarbons that contain only carbon-carbon single bonds and carbon-hydrogen bonds.

General formula: CnH2n+2

• No functional group. There r only C—C and C—H bonds.
• suffix: —ane

Question 3

Alkenes

Answer 3

Alkenes are hydrocarbons that contain carbon — carbon double bonds (C=C).

General formula: CnH2n

Functional group:
Carbon — carbon double bond (C=C)
Suffix: —ene

Question 4

Alcohols

Answer 4

Functional group:
Hydroxyl group (—O—H)
Suffix: —ol

Question 5

Carboxylic acids

Answer 5

Functional group:
Carboxyl group
Suffix: —oic acid

Question 6

Number of carbon atoms and their prefixes

Answer 6

1: meth—/methan—
2: eth—/ethan—
3. Prop—/propan—
4. But—/butan—

(Note: Alkenes homologous series dont have ‘methene’ as alkene’s functional group is carbon—carbon double bond thus u nd 2 carbons)

Question 7

Alkanes as saturated hydrocarbons

Answer 7

Saturated hydrocarbons r hydrocarbons that contains only single covalent bonds betw the carbon atoms

Question 8

Physical properties of alkanes

Answer 8

1. Low melting and boiling points.
   - Alkanes r simple molecular substances w weak intermolecular forces of attraction.
   - generally increases down the homologous series
   ——> down the homologous series, size of the molecule increases,resulting in stronger intermolecular forces of attraction thus more energy is needed to overcome it.
2. Viscosity
   Becomes more viscous as their molecular sizes increase due to stronger intermolecular forces of attraction
3. Solubility
   Alkanes are insoluble in water but soluble in most organic solvents such as tetrachloromethane (CCl4). Liquid alkanes r also often used as solvents for other organic compounds.

Question 9

Chemical properties of alkanes

Answer 9

Generally is reactive as they only consist of strong carbon-carbon single bonds and strong carbon-hydrogen bonds which r hard to break. However, they undergo combustion and substitution reaction.

Question 10

Combustion of alkanes

Answer 10

In excess oxygen, complete combustion of alkanes occur.

Alkane + oxygen -> carbon dioxide + water

(Water is in gaseous state during combustion and liquid when cooled)

In insufficient oxygen, incomplete combustion takes place. Instead of carbon dioxide, carbon monoxide and soot is formed.

Alkane + oxygen -> carbon monoxide + carbon + water

Question 11

Substitution reaction in alkanes

Answer 11

Alkanes react w halogens in presence of ultraviolet light.

[A hydrogen atom is substituted by a x atom to form y compound.]

Question 12

Unsaturated hydrocarbons

Answer 12

Are hydrocarbons that contain double covalent bonds between the carbon atoms

Question 13

Physical properties of alkenes

Answer 13

1. Low melting and boiling points
   - as alkenes r simple molecular substances w weak intermolecular forces of attraction.
   - There is a gradual increase down the homologous series.
2. Viscosity
   - more viscous as their molecular sizes increases, exulting in stronger intermolecular forces of attraction
3. Solubility
   Insoluble in water but soluble in most organic solvents like tetrachloromethane (CCl4)
   - they’re more reactive than alkanes and might interfere in reactions thus not usually used as solvents
   - more often used as starting materials in the manufacture of various products eg plastics and detergents

Question 14

Combustion of alkenes

Answer 14

In excess oxygen, complete combustion occurs

CnH2n + O2 -> carbon dioxide (g) + H2O (g)

(Chemical Eqn is not balanced, just general terms)

Percentage by mass of carbon in Alkenes molecules is higher than in alkanes thus more likely to undergo incomplete combustion, thus producing carbon monoxide and carbon. Hence, they burn w a sootier flame than alkanes.

Alkanes + oxygen -> carbon monoxide + carbon + water

Question 15

Addition of hydrogen to alkenes (hydrogenation)

Answer 15

Reagent: H2
Conditions: 150dgC, nickel catalyst

Application: manufacture of margarine by the hydrogenation of vegetable oil

As more hydrogen molecules r added into the oil molecule, there r fewer carbon-carbon double bonds and the melting point of the oil increases. Trans-fat may be formed due to the side reaction of the hydrocarbon chain w the catalyst.

Question 16

Polysaturated fats

Answer 16

Contain hydrocarbon chains w 2 or more carbon-carbon double bonds in each chain.

The greater the level of saturation, the higher the melting point of the substance

Question 17

Fats vs oil

Answer 17

Fats: solids at rtp
Oils: liquid at rtp

Fats: contain mainly saturated fat molecules
Oils: higher percentage of unsaturated fat molecules

Question 18

Addition of bromine to alkenes (bromination)

Answer 18

Alkenes can react w halogens. When ethane is bubbled thru liquid bromine, the red-brown colour of bromine disappears (decolourisation). A colourless liquid (1,2-dibromoethane) is formed.

Question 19

Addition of steam (hydration)

Answer 19

Alkenes can react w steam to produce alcohols.
Reagent: steam H2O
Conditions: 300dgC, 60atm, phosphoric(V) acid, (H3PO4) catalyst.

Question 20

Addition polymerisation

Answer 20

Polyethene can be formed by high tempt + pressure + catalyst, as ethane react w each other.

Conditions: high tempt & pressure, catalyst

Used to manufacture materials like plastic.

Question 21

Cracking

Answer 21

A process in which larger hydrocarbon molecules, usually alkanes, r broken down into smaller hydrocarbon molecules

Question 22

Catalytic cracking

Answer 22

Conditions:
1. Presence of a catalyst such as aluminium oxide (Al2O3) and silicon dioxide (SiO2) (broken pot)
2. High tempt of about 500dgC to 700dgC
3. Pressure of about 1 atm

General equation:
Long-chain alkanes —(catalytic cracking)-> (mixture of short-chain alkenes) + (mixture of short-chain alkanes or hydrogen gas)

Question 23

Importance of cracking

Answer 23

Cracking converts the less useful components of crude oil, usually long-chain alkanes, into
1. More useful shorter-chain alkenes such as ethane and propene, which r starting materials for many important industrial processes
2. Higher demand short-chain alkanes such as petrol and fuel
3. Produces hydrogen as a byproduct used in impt industrial reactions eg production of ammonia

Question 24

Isomers

Answer 24

Compounds that hv the same molecular formulae but diff structural formulae

1. Molecular formula: constant
2. Structural formula: varies
3. Boiling point: varies
4. Combustion products: same moles same products
5. Hydrogenation: same molecule produced
6. Polymerisation: different product

Isomerism can be obtained thru changing the position of the functional group or shifting the position of the alkyl groups (CH3)

Question 25

Test for unsaturation (alkenes)

Answer 25

Aq bromine is a red-brown solution formed when bromine dissolves in water

Aq bromine remains red-brown when shaken w alkanes, decolourises when shaken w alkenes.

Question 26

X is a gas at rtp that burns to produce carbon dioxide and water. What can you tell?

Answer 26

X is a hydrocarbon with small carbon chain that has less than 5 carbon atoms for it to be a gas at rtp. Therefore, X is methane.

Question 27

Nylon vs Terylene

Answer 27

Nylon is a macromolecule that has small units joined by amide linkages. Terylene is a macromolecule that has small units joined by ester linkages.

Question 28

Are products x and y isomers of h? Explain your reasoning. [2]

Answer 28

Product X is not an isomer as it has a different molecular formula than h. Product 2 is an isomer as product 2 has the same molecular formula as y but different structural formula.
Isomers are compounds with the same molecular formula but different structural formula

Question 29

Processes used to manufacture alkene

Answer 29

Fractional distillation of crude oil followed by catalytic cracking

Question 30

Process used to manufacture polymer

Answer 30

Addition polymerisation of monomer (alkene or alkane)

Question 31

Process used to manufacture ethanol

Answer 31

Process 1: catalytic addition of steam to ethene

Process 2: fermentation of glucose

Question 32

Process used to manufacture carboxylic acid

Answer 32

Oxidation of corresponding alcohol

Question 33

Use the info given in table to give 2 pieces of evidence that suggests that the aldehydes are a homologous series. [2]

Answer 33

Each member of the aldehyde homologous series differs from the next by CH2. The aldehydes have the same functional group, –CHO. Their molecular formulas can be reduced to the same general formula.

Question 34

Give a similarity and a difference between the bonds in these vegetable oils and alkenes. [2]

Answer 34

S: both contains carbon-carbon double bonds

D: Alkenes have 1 carbon-carbon double bond in each molecule but polyunsaturated vegetable oils have 2 or more.

Question 35

Describe the manufacture of margarine from vegetable oils [2]

Answer 35

Hydrogenation of vegetable oil where hydrogen as is passed through vegetable oils at 150dgC with nickel catalyst

Question 36

Explain why propane and butane leave the column in the same fraction. [1]

Answer 36

They have a similar number of carbon atoms and a similar range of boiling points

Question 37

Suggest reasons why the enthalpy change of combustion of octane is more negative than the enthalpy change of combustion of butane. [3]

Answer 37

More energy is released to form C-O and O-H bonds in CO2 and water after the combustion of one mole of octane as compared to amount of energy released to form bonds in CO2 and H2O after the combustion of one mole of butane.
Octane contains a larger carbon composition than butane and thus has more C-H bonds to be broken.

Question 38

Explain why carboxylic acid turns Universal Indicator solution orange. [1]

Answer 38

Carboxylic acid is an organic acid and thus is a weak acid with pH 3.